Physics Flashcards
Cathode is also called the?
Filament
Anode is also called?
Target
The cathode is what charge?
Negative
The anode is what charge?
Positive.
What is the purpose of the oil in an x-ray tube?
Electric shielding X-ray attenuation/protection Transmission of heat
What percentage of the kinetic energy of the electrons colliding into the target gets converted to x-rays?
1%
What are the three main ways to cool an x-ray tube?
Brute force - add more material to the anode Material approach - Add rhenium to the focal track. Add graphite or molybdenum to the anode disk for more heat capacity. Use liquid metal vacuum bearings for faster rotation of the anode. Add circular grooves in the anode. Add more rows to reduce scan duration (indirect) Paradigm Shift - new technologies (example dual-source)
How many collimators does a CT have?
Two: Tube collimator and detector collimator
What defines the slice thickness in a single slice CT?
Tube collimator
What does a filtration do?
Filters out low-energy x-rays that contribute to patient dose and scatter
What are filters normally made from?
Aluminium ~3mm
What is the advantage of a bow-tie filter?
Allows a lot of x-rays to pass through the center of the patient and less to pass through the thinner margins of the patient.
What type of detectors are used in CT?
Scintillation detectors
What material is used in a scintillation detector?
Crystals - cesium iodide or cadmium tungstate
What is another name for the third generation gantry?
Rotate/rotate - meaning the both the x-ray tube and detectors rotate
When was CT invented?
1972
Why was fourth generation CT created?
To get rid of the ring artifact
What is the fourth generation CT?
Stationary detectors at 360 degree - very high cost
What is the collimator pitch in a single slice system?
Ratio between the table movement per 360 degrees of rotation to collimator width (single slice CT)
What is the detector pitch?
Ratio between the table movement per 360 degrees of rotation to detector width (single slice CT)
What is the collimator pitch in a multi slice system?
Ratio of the detector pitch to the number of detector rows
What does the pitch influence?
Dose Scan time Image quiality
What is interpolation?
Reconstructing the CT image and filling in the blanks in helical scanning
What do collimators determine in single slice vs multislice CT?
Single slice = detector width Multi slice = number of slices
What is the table made out of to reduce artifacts?
Carbon fiber
What is the scan field of view?
The area where there is complete overlap of the wedges of exposed area
What is display field of view?
It is the area of the SFOV where the image is reconstructed. This determines pixel size.
Slice thickness is proportional to the what?
The magnitude of the volume averaging
Slice thickeness is inversely proportional to what?
The magnitude of noise
Structures with wide inherent object contrast (bone, lung) should be viewed with what type of window?
Wide window… this suppresses noise. Therefore small slices are used for orthopedics because they can handle the noise
Structures with narrow inherent contrast (brain, liver, spinal cord) show be viewed in?
A narrow window which enhances noise. Therefore 3-5mm slices are good cause they decrease noise and in crease signal
What is this artifact? How does the nyquist theory play into it?
Aliasiing.. specifically ray aliaising. Nyquist theory states that you must have at least two samplings per wave length.
How is aliasing created?
Insufficient data sampling leading to data ambiguity
What are the two types of aliasing?
Ray
View
How to remedy aliasing?
Avoid high frequency/dense objects
Quarter detector offset (where the center of the beam is offset by 1/4 of a detector)
Dynamic focal spot (focal spot wobbles back and forth between two positions)
Reduce rotation time
What artifact is this?
Motion
Slice mismatch
Blurring
Ghosting
Streaks
How do you correct for motion?
Underscanning - only collect datat from part of the tube revolution
Overscanning - The tube rotates 400 degrees and
What artifact is this?
Photon starvation
What is photon starvation/streaking?
When you go through a highly dense area and it starves the beam of the appropriate amount of photons. Basically blocks the beam.
What is this artifact?
Beam hardening
What is this artifact?
Beam hardening
What is this artifact?
Beam Hardening
How does beam hardening happen?
You block the low energy and therefore there is increased average beam energy. This causes streaking because your beam passes through structures without being attenuated and therefore makes it look like a less dense structure than it is.
What is this artifact?
Volume averaging
What is this artifact?
Ring artifact
What is this artifact?
Ring artifact
How does the ring artifact happen?
Miscalibration of a detector or detector fails
What is this artifact?
Stair step
What is this artifact?
Metal artifact
